Food web structure is an important indicator of ecosystem health because it reflects both species richness and functional diversity. Recent attempts to use compound-specific stable isotopes rather than bulk-tissue techniques to evaluate food webs promise to improve understanding of food web dynamics and trophic interactions. These new methods, which rely on amino acid isotope analyses, need to be validated experimentally before this promising advance can be applied to natural food webs. This study relies on a laboratory multi-level food chain experiment to compare the ecological sensitivity and the costs and benefits of new amino acid techniques and common bulk-tissue methods. The researchers will create a realistic, four-level food chain in the laboratory that allows consumers to integrate prey isotopic signatures into their tissues. At the end of the experiment, both amino acid and bulk tissue stable isotopes will be analyzed to determine whether amino acids provide a more accurate measure of trophic position than bulk tissue methods.
The broader impacts of this project lie primarily in advancing a new method for evaluating natural food webs. If successful, the method could eventually be applied to terrestrial and aquatic communities, and would advance the general field of food web ecology. It also could be applied to museum specimens, allowing researchers to examine historical as well as contemporary food webs.
Determining the health of any aquatic or terrestrial environment has traditionally involved collecting either indirect data on abiotic conditions (for example, ambient temperatures and concentrations of potential pollutants such as inorganic nutrients, pesticides, and herbicides) or direct data on diversity and abundance of organisms. The latter is more reflective of the actual state of the community of organisms living within a particular habitat, but collecting these data is very time-consuming. One solution has been to evaluate parameters describing overall community functioning and health using measures such as food web structure, including trophic position (TP) and maximum TP (= food chain length, or FCL). Over the last few decades, FCL has often been calculated using stable isotopes of carbon and nitrogen. For example, the greater the concentration of the rarer heavy nitrogen (15N) over the abundance of the more common lighter nitrogen (14N), the higher the trophic position of the organism in the food web. Traditional approaches to measuring FCL in aquatic communities have involved using what is known as bulk-tissue stable isotope analysis. In this approach, whole tissue (such as a sample of fish muscle) is analyzed in the top consumer (for example, a walleye fish). This approach also requires that the investigator evaluate the isotopic signature of the basal food source (such as algae or a surrogate such as a primary herbivore like a grazing snail) to determine how many steps of the food ladder the top predator is located. The current project evaluated the advantages and disadvantages of using a more refined technique that involved measuring isotope ratios of nitrogen in the tissue’s amino acids rather than in the whole tissue. Consumer tissue contains some amino acids which change at each step up the food chain and others that do not change. The results of the NSF study demonstrated a highly significant increase in accuracy and precision of the measurements of FCL. This will then permit the investigator to collect fewer samples of the target species (e.g., the walleye) and to eliminate collecting the basal food source. This project also demonstrated a significant effect of starvation (which increases the tissue’s ratio of 15N to 14N) on interpretation of FCL.
